The present invention relates to methods for manufacturing screen cylinders and flat screen plates for use, for example, in the pulp and paper industry for screening pulps and to the cylindrical and flat screens formed thereby.
In the formation of paper products from pulp, the pulp is screened such that impurities, such as sticks and other undesirable pulp constituents, are removed. This screening process effectively separates the accept and reject portions of the pulp. Screening is generally performed on flat plates or screen cylinders, each of which is provided with openings therethrough for separating the accept and reject portions of the pulp.
In many such screens, whether flat or cylindrical, alternating grooves and ridges are provided in one of the surfaces of the screen. For example, see U.S. Pat. No. 4,529,520. These ridges and grooves are provided for adjusting the flow characteristics of the pulp passing through the screen. While these grooves and ridges have proven to be highly desirable, screens and cylinders with such surface configurations have not heretofore been readily and easily manufactured. For example, such screens are frequently formed by first providing a plurality of openings through a flat metal plate. A plurality of bars are then secured to the plate at longitudinally spaced positions therealong in parallel alignment one with the other to form the ridges and the grooves therebetween. Particularly, the bars are conventionally welded along their opposite sides to the surface of the plate. The fabrication time and, hence, the cost of manufacture of screen plates of this type is quite substantial. Additionally, and importantly, the welds on the opposite sides of the bars occupy substantial space on the surface of the plate. The apertures through the plate are therefore oftentimes blocked by the welds and the number of screen apertures, holes or slots, is accordingly diminished. Welds along the bars also causes stress risers in the screen plate which is subject to heavy vibrations caused by high frequency pulses. Stress cracks and premature failures have often been noticed in the described areas.
It will be appreciated that it is not simply a matter of increasing the number of apertures through the screen to compensate for such reduced numbers because predetermined spacings between the apertures, holes or slots must be maintained to avoid the stapling phenomena which clogs the screen plate. Consequently, there has been demonstrated a need for a method of manufacturing screen plates of this type for use in the pulp and paper industry wherein substantial reductions in cost and manufacturing time can be achieved, as well as providing screen plates, in both flat or cylindrical form, which are effective to screen pulp with increased efficiency and capacity.
Therefore, in accordance with the present invention, there is provided a method of manufacturing a metal screen plate wherein manufacturing costs and fabrication times are greatly reduced in comparison with prior art methods of forming similar-type plates. Particularly, the present invention provides a plate having an initial thickness corresponding to the full thickness dimension of the resulting plate, including the ridges. To form the screen apertures, blind holes or slots are formed through one face of the plate to a predetermined depth short of the opposite face of the plate. These apertures are formed at longitudinally and transversely spaced locations along the plate. Larger diameter concentric holes are then formed using the smaller diameter holes as pilot holes. These larger diameter holes are provided similarly to a predetermined depth but less than the predetermined depth of the smaller diameter holes. The opposite face of the plate is then machined to form longitudinally extending rows of grooves with ridges (bars) therebetween. Thus, the opposite face is machined to remove rows of plate material to a depth to expose the termination of the smaller diameter holes in the metal plate. That is, the material on the opposite face is machined to the extent that the smaller diameter holes open through the plate but only in the areas of the rows of grooves between the ridges.
It has also been found that when producing the sizing holes by drilling the present invention creates less burrs in the holes compared to conventional drilling methods with drills "breaking-through".
In certain applications it is advantageous to angle one of the side faces of the ridges (i.e., make "profiled" bars) and this can be accomplished simply by machining the side faces of the ridges to the appropriate angle. In this manner, one side of the groove is formed by a perpendicular face of a ridge, while the opposite side of the groove is defined by an angled face of the next-adjacent ridge and which angled face extends away from the base of the slot. It will be appreciated that the number of apertures through the plate at like spacing is substantially increased in comparison with the apertures provided through similar plates of the previously described prior art construction. Additionally, the manufacturing process according to the present invention may provide for an increase in effective screen area of approximately 40% with respect to prior processes in a similarly sized plate. Further, the screen plates of the present invention may be manufactured with costs and fabrication times reduced by approximately one-half. The screen plates resulting from the foregoing-described method may be used in their flat plate configuration or may be rolled to form a screen cylinder.
In the description of the invention in the specification and claims, the term "openings" will be used throughout. This term is used for convenience only, and is intended to encompass apertures of all shapes and sizes, including holes, slots, orifices and passageways.
In a preferred embodiment of the present invention, there is provided a method of manufacturing a metal screen plate having first and second opposite faces, comprising the steps of (a) forming openings through the first face and into the body of the metal screen plate a predetermined distance to terminate within the plate body at a position P short of the second face and (b) machining the second face to remove metal material along the majority of the surface thereof to a depth to expose the position P so that the openings extend entirely through the plate body, and leave a plurality of ridges in the second face spaced one from the other therealong after machining is completed.
In a further preferred embodiment of the present invention, there is provided a method of manufacturing a screen plate having first and second opposite faces, comprising the steps of (a) forming openings through the first face and into the body of the screen plate a predetermined distance to terminate Within the plate body short of the second face and (b) removing material from the second face in longitudinally extending generally parallel rows thereof along the majority of the surface thereof to a depth at least equal to the depth of the plate less said predetermined distance to form a plurality of grooves in the second face so that the openings extend entirely through the plate body and open into the grooves, but leave a plurality of ridges in the second face spaced one from the other on opposite sides of the grooves after material removal is completed.
Accordingly, it is a primary object of the present invention to provide a novel and improved method of manufacturing screen plates for use in the pulp and paper industry, and screen plates thereof, and which affords substantial reductions in the cost and fabrication time required for their manufacture in comparison with prior screen plates of similar types.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.